The present invention is related to improved solid electrolytic capacitors comprising conductive polymeric cathodes. More specifically, the present invention is related to improvements in the conductive polymer wherein the improved polymer composition provides improved coverage, particularly on edges and corners, thereby providing a capacitor with improved ESR and improved leakage stability in humid environments.
Solid electrolytic capacitors are widely used throughout the electronics industry. In high voltage applications capacitors with a solid electrolyte, formed by conductive polymer dispersions, give excellent high voltage performance compared to conductive polymer cathodes formed in-situ. These conductive polymer dispersions are prepared by a number of process steps including polymerization, purification, filtration, homogenization, evaporation, etc. Descriptions of these processes are provided in U.S. Pat. Nos. 5,300,575; 7,990,684; 7,270,871; 6,000,840 and 9,030,806; U.S. Patent Publication No. 2011/0049433 and PCT Publication WO 2010/089111 each of which is incorporated herein by reference.
Capacitors and methods of making capacitors are provided in U.S. Pat. Nos. 7,990,683; 7,754,276 and 7,563,290 each of which is incorporated herein by reference.
Solid electrolytic capacitors comprising conducting polymer, as the cathode, have several disadvantages. One disadvantage is the difficulty associated with covering edges and corners of the dielectric. Poor coverage of conducting polymers on corners and edges of anodized anodes results in high DC leakage current and causes reliability problems in humid atmosphere.
Equivalent Series Resistance (ESR) stability of the capacitors requires that the interface between the cathode layer, cathodic conductive layers, conductive adhesive, and leadframe have good mechanical integrity during thermo mechanical stresses. Solid electrolytic capacitors are subject to various thermomechanical stresses during assembly, molding, board mount reflow etc. During board mount the capacitors are subjected to temperatures above 250° C. These elevated temperatures create stresses in the interfaces due to coefficient of thermal expansion (CTE) mismatches between the interfaces. The resultant stress causes mechanical weakening of the interfaces. In some cases this mechanical weakening causes delamination. Any physical separation between the interfaces causes increases in electrical resistance between the interfaces and thus an increased ESR in the finished capacitor. This interfacial weakness also results in higher ESR shift during high humidity environment.
European Patent Application EP-A-1746613 improves the process of forming solid electrolytic capacitor from EP-A-1524678 by virtue of solid particles having a diameter in the range from 0.7 to 20 μm being added to the dispersion. The solid particles are particles of electrically conductive polymer or fillers such as carbonates, silicates, silica, calcium sulphate, barium sulphate, aluminium hydroxide, glass fibres, glass bulbs, wood flour, cellulose powder, carbon black, silicon oxides or silicon dioxide. The patent claims improved edge and corner coverage by addition of the aforementioned particles in conductive polymer polyanion dispersion. However, the solid particles of conductive polymer does not contain polyanion which makes it non-disperive or insoluble in water. Thus, the addition of solid particles of conducting polymer in conducting polymer:polyanion dispersion can affect dispersion stability, causes the dispersion to have a very high viscosity, setllements of solid particles in the dispersion, and results in poor reproducibility in performance. Moreover, as mentioned in US Patent Pub. No. 2015/0140203 A1, the solid particles makes the polymeric outer film brittle, which can cause the outer layer to flake off locally resulting in an increase in the residual current and in the ESR.
To improve coverage without affecting polymeric outer layer film strength, WO2010089111A1, which is incorporated herein by reference, reported the use of a group of chemical compounds, referred to as crosslinkers or primers, which are mostly multi-cationic salts or amines. The crosslinker is applied to the anodized anode prior to the application of polymer slurry to achieve good polymer coverage on corners and edges of the anodized anode. The use of crosslinker eliminated the need of solid particles in conducting polymer dispersion for coverage improvement. The effectiveness of the crosslinker is attributed to the cross-linking ability of multi-cationic salts or amines to the slurry/dispersion particles. While crosslinkers are advantageous for improving the coating coverage on corners and edges of the anodized anode, the addition of these crosslinkers, which are mostly ionic in nature, has the unintended consequences of degrading the performance under humidity such as high ESR shift and increased DC leakage in a finished product.
There was been an ongoing need for an improved conductive polymer capable of achieving better corner and edge coverage in a solid electrolytic capacitor, and process for forming the capacitor, without degrading the ESR and leakage reliability performance in humid conditions.